According to prior art, hydraulics is used for controlling various actuators, the most common being cylinders and motors. In hydraulics, substantially incompressible hydraulic fluid is utilized, whose more precise composition varies and which is used as a pressurized medium. The volume flow of hydraulic fluid is used to produce a linear movement of a cylinder, when the hydraulic fluid is led into a chamber accommodating a movable piston and a piston rod, or a rotary movement of a motor, when the hydraulic fluid is led through the motor. The cylinder and the motor have a speed of movement and a speed of rotation, respectively, that is dependent on the volume flow. The piston of the cylinder also has a force that is dependent on the pressure of the hydraulic fluid supplied into the chamber. The shaft of the motor has a torque that is dependent on the pressure of the hydraulic fluid effective over the motor. In this way, it is possible to control various apparatuses at a desired speed and with a desired force or torque.
The volume flow and pressure of the hydraulic fluid are produced in a known way by means of an apparatus that is most commonly used as a pump. In many cases, the same apparatus can be used as a motor or a pump, on whose shaft a driving device is mounted, usually an electric motor that rotates the apparatus. By means of a hydraulic pump, mechanical energy (torque, rotational speed) is converted to hydraulic energy (volume flow, pressure). There are several types of pumps, but in this case, especially the axial piston pump and the radial piston pump are mentioned, which operate by the principle of displacement.
The above-mentioned pumps comprise several chambers operating in cycles and with a phase shift with each other, wherein, as their sum, an almost steady volume flow is achieved on the pressure side of the pump and in its outlet. A piston moves back and forth in the chamber in a sealed manner, sucking hydraulic fluid into the expanding chamber from the suction side and the suction inlet through a suction valve that is opened by underpressure. As the piston reduces the volume of the chamber, the pressure of the hydraulic fluid is increased and it is led onto the pressure side via a pressure control valve that is opened by pressure. By controlling the movement of each piston in such a way that the volume displaced by it varies, it is also possible to control the total volume flow produced by the pump.
The pressure side of the pump can be coupled via a suitable control valve, for example, to the cylinder chamber or motor. From the cylinder and the motor, the hydraulic fluid is transferred further to a tank line or a tank connected to the suction side of the pump.
FIG. 1 shows a system of prior art, comprising a pump and two actuators, as well as two control valves. This is a so-called load sensing (LS) system with an adjustable displacement pump. The volume flow produced by the pump can be divided between two cylinders. The volume flow is dependent on the pressure difference effective over the control valve and on the position of the control valve. However, such systems involve the problem that the pressure of the whole system is determined by the actuator requiring the highest pressure. For this reason, the pressure is unnecessarily high for the other actuators, resulting in unnecessary pressure losses and wasting of energy.
FIG. 2 shows a system, in which each actuator is controlled by a separate adjustable displacement pump, wherein also their pressure sides are unconnected. The pressures of the system can be selected separately for each actuator, but the problem is that several pumps are needed, which increases the costs and the size of the system.
Document EP 1537333 B1 discloses a pump, by which actuators can be controlled in a more versatile way. The inlet valve and the pressure control valve relating to each capacity can be electrically controlled so that each piston can be entered in various modes. In the different modes, each piston is either running idle, in which case it does not produce a volume flow or pressure, or each piston may produce a varying quantity of volume flow depending on how long and in which phase of the piston movement the pressure control valve is open. Consequently, the volume flow produced by the piston can be led either partly or in whole back via the inlet valve. By means of said function, it is possible to control the total volume flow produced by the motor. The control is implemented by means of an intelligent valve-controlling control device according to the volume flow required at each moment.
Using the pump of EP 1537333 B1, it is not possible to eliminate the problems relating to the high pressure level shown in FIGS. 1 and 2, even if the volume flow produced by the pump can be controlled on the pressure side. Document U.S. Pat. No. 6,681,571 B2 also discloses an apparatus used as a pump and a motor, having two ports which can be used either as pressure inlets or suction inlets. The chambers operating by the displacement principle comprise two electrically controlled valves for controlling the quantity and direction of the volume flow. When part of the chambers is not in operation, the valves being closed, it is possible to control the total volume flow produced by the apparatus.